Apparatus and method for securing end pieces to a mandrel

ABSTRACT

A method for locking an internally threaded end piece such as a head piece, a tail piece, an end or sealing nut to a mandrel for a down hole tool to obviate the need for pins, set screws, and the like, to provide a more robust assembly. In two embodiments the threaded end piece is locked to the mandrel in a swaging operation.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/996,828, filed Jan. 15, 2016, entitled ROBUST BUMPER SPRINGASSEMBLY, and claims priority to U.S. Provisional Patent ApplicationSer. No. 62/103,921 filed Jan. 15, 2015 by the same inventors with thesame title.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to tools for use in oil and gaswell operations, and more particularly to improved bumper spring toolsfor use in oil and gas wells.

2. Background of the Invention and Description of the Prior Art

A newly drilled and completed well typically has enough pressure withinthe formation to cause liquids in the formation and the well to flow tothe surface without aid. Over time, however, as the well's productionvolume and bottom-hole pressure decline the liquids fall back on theperforations—the passages into the formation—thus creating what iscalled a “loaded well” condition. In this condition the well no longerhas sufficient pressure to cause the liquids to flow to the surfacewithout some artificial lift.

A plunger lift is a type of artificial lifting device utilized in oiland gas wells to efficiently unload liquids. The system usually requiresno external energy to provide the necessary pressure to lift the liquidsto the surface, instead relying on the residual pressure in the well tolift the plunger. The gas-to-liquid ration required varies depending onmany conditions. The common rule of thumb used in the industry is 300 to400 scf per barrel per 1000′ of depth.

FIG. 1 illustrates a conventional oil and gas well fitted with a plungerlift system for controlling production, including generic plunger liftand bumper spring devices typical of the prior art. The systemillustrated in FIG. 1 typically comprises the following structures. Awell 10 is formed by a casing 12 that lines the well 10. Within thecasing 12 is a tubing string 14 that encloses S the well bore 16 throughwhich oil or gas 30 is produced from a formation 18 through perforations20. Within the well bore 16 is a bumper spring assembly 22 resting on aseating nipple 24 (which may also be called a tubing or collar stop). Alift or bypass plunger 26, shown traveling upward under the pressure ofthe fluids and/or gas in the well bore 16, pushes or lifts a “slug” offluid 32 ahead of it. The well 10 includes the wellhead apparatus 42disposed on the surface of the earth 40 for directing the production ofthe well to appropriate receptacles or pipelines (not shown).

A bumper spring assembly is a tool that is typically placed in a seatingnipple at the lower end of the tubing in the well to absorb the momentumof the bypass or lift plunger as it reaches the seating nipple, therebyprotecting the seating nipple from damage. Structurally, most bumpersprings comprise a shaft or mandrel, a head piece at the upper end and acage attached to the lower end. The head piece and cage are typicallythreaded onto the mandrel and secured with a pin to prevent the rotationof the end piece with respect to the mandrel so that the bumper springbecomes disassembled. Other methods to prevent loosening of the endpieces include welding and lock nuts.

Conventional plungers do not readily fall through flow within the well,so the well must be closed to stop the flow so that the plunger willfall to the bottom of the well due to gravity, contacting the bumperspring at the bottom. The fall speed of such plungers (which includepad, brush, solid, sand, spiral, etc.) typically ranges from 50 to 400feet/minute. Newer types of plungers (such as bypass, continuous run,flow-through, ball & sleeve, sliding sleeve, etc.) are designed forfalling through flowing liquids to enable substantially high productionfrom the well. This performance is provided by features such as passagesor ports machined into the body of the plunger or its cage to permitliquid flow. However, the rate of fall of these plungers may reachvelocities as high as 2000 feet/minute. The greater momentum of suchplungers places much greater stress on the pinned components of thebumper spring. This is a serious disadvantage because the pins arefrequently sheared, resulting in loosening of the components of thebumper spring. The head piece or cage may become unscrewed, allowingloose pieces to travel up and down the well depending on the flow of thewell. Damage to the plunger, the well casing or other structures mayresult; in other cases the plunger may become stuck in the well bore,lodged there by pieces that become wedged between the plunger and thewell bore. This causes the well to be shut down while the problem isrepaired, causing a substantial loss of production.

Conventional bumper spring assemblies may also impede the flow ofproduction because of their position within the well bore, often in thepath of fluids entering the well bore from the perforated regions of thewell casing.

Accordingly there is a need for an improved bumper spring assembly thatsurvives many high-velocity cycles in the well bore and presents minimalobstruction to the flow of production facilitated by the bypass or liftplunger used to restore production.

SUMMARY OF THE INVENTION

Accordingly in a first embodiment of the present invention a bumperspring assembly is provided comprising a one piece mandrel having a headend and a tail end, an integral head piece at its head end, and aconcentric bore formed in the tail end that further includes an externalthread surrounding the tail end; a bumper spring configured as a coilspring disposed concentrically on the mandrel, a first end thereofadjacent the head end; a bumper spring cage disposed concentrically onthe mandrel adjacent a second opposite end of the bumper spring; and acage nut, having an internal screw thread terminated by anoutward-directed chamfer near the distal end of the cage nut, threadedonto the external screw thread of the tail end of the mandrel therebysecuring the bumper spring and the bumper spring cage between the headend of the mandrel and the cage nut; wherein further a distal portion ofthe tail end of the mandrel is expanded outward in a cold formingprocess against the threads in the cage nut next to the outward chamferto lock the threads of the cage nut to the tail end of the mandrelwithout the use of pins or set screws.

In another aspect of the first embodiment, the outward expansion of thedistal portion of the tail end of the mandrel deforms the endmost screwthreads of the cage nut and the tail end of the mandrel proximate theoutward-directed chamfer.

In another aspect, the headpiece comprises a solid cylindrical memberconfigured with a fishing hook at a first end and a stabilizing body ata second end wherein the stabilizing body includes longitudinalstabilizing ridges, concentric flow reliefs, and chamferred edges.

In another aspect, the longitudinal stabilizing ridges comprise aplurality of longitudinal strips of the outermost diameter of thestabilizing body alternately disposed between the longitudinalundercuts.

In another aspect, the concentric flow reliefs comprise a plurality oflongitudinal undercuts in the outermost diameter of the stabilizingbody, alternately disposed with the longitudinal stabilizing ridgesaround the circumference of the stabilizing body.

In another aspect, the longitudinal undercuts are of substantially equaldimensions and may be either flat or curved in a concave direction.

In another aspect, the bumper spring comprises a coil spring formed ofwire having a uniform pitch that may include closed and ground ends.

In another aspect the bumper spring cage is a hollow cylindrical memberthat may include one or more of flow ports, relieved channels, andegress sweeps, wherein the flow ports may include at least two elongatedopenings through the walls of the hollow cylindrical member that areseparated by substantially equal intervals around the circumference ofthe bumper spring cage.

In a second embodiment of the present invention a mandrel for a bumperspring assembly is disclosed comprising a one piece mandrel having ahead end and a tail end, an integral head piece disposed at its headend, and a concentric bore formed in the tail end that further includesan external thread surrounding the tail end; and a cage nut having aninternal screw thread is threaded onto the external screw thread of thetail end of the mandrel for securing a bumper spring and a bumper springcage between the head end of the mandrel and the cage nut; whereinfurther a distal portion of the tail end of the mandrel is swaged duringa cold forming process to lock the cage nut to the tail end of themandrel without the use of pins or set screws.

In another aspect, the swaged distal portion of the tail end of themandrel is flared against the cage nut.

In another aspect, the integral headpiece comprises a solid cylindricalbody having first and second ends and a fishing neck disposed at thefirst end thereof; a stabilizing body disposed at the second end of theheadpiece and having a plurality of stabilizer ridges disposed around afill diameter thereof; wherein the stabilizing body includes a pluralityof undercut reliefs disposed in alternating relationship with thestabilizer ridges around the circumference of the headpiece and parallelto a longitudinal axis of the headpiece; and a taper or chamfer aroundedges of the stabilizing body proximate the second end of the headpiece.

In another aspect, a bumper spring cage for use with the mandrel of thesecond embodiment comprises a hollow cylindrical member having aplurality of elongated flow ports formed between an inside bore and anouter surface of the hollow cylindrical member; wherein the elongatedflow ports are uniformly disposed around the circumference of the hollowcylindrical member.

In another aspect, the bumper spring cage comprises a plurality of exitchannels connecting the elongated flow ports with the outer diameter ofthe cage; wherein the exit channels are fluted with a plurality oflongitudinal grooves along the length thereof.

In another aspect, a bumper spring for use with the second embodimentcomprises a coil spring disposed concentrically on the mandrel, whereinthe coil spring is formed of wire having a uniform pitch, and mayinclude closed and ground ends thereof.

In other embodiments, a method for locking an internally threaded endpiece such as a head piece, a tail piece, an end or sealing nut to amandrel for a down hole tool is provided to obviate the need for pins,set screws, and the like, to provide a more robust assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a typical oil and gas well fitted with a prior artplunger lift and bumper spring devices;

FIG. 2 illustrates an exploded view of a bumper spring according to oneembodiment of the present invention;

FIG. 3 illustrates a detail view of the head end of the embodiment ofFIG. 2;

FIG. 4 illustrates a detail view of the cage end of the embodiment ofFIG. 2;

FIG. 5A illustrates an assembled side view of the bumper springembodiment of FIG. 2;

FIG. 5B illustrates a cross section view of the assembled bumper springof FIG. 5A;

FIG. 6 illustrates a cross section view of the cage end of oneembodiment of the present invention prior to a swaging operation;

FIG. 7 illustrates a cross section view of the cage end of theembodiment of FIG. 6 after the swaging operation;

FIG. 8 illustrates a side view of a swaging tool for cold forming theembodiment depicted in FIG. 7;

FIG. 9 illustrates a cross section view of the cage end of a secondembodiment of the present invention prior to a swaging operation;

FIG. 10 illustrates a cross section view of the cage end of theembodiment of FIG. 9 after the swaging operation; and

FIG. 11 illustrates a side view of a swaging tool for cold forming theembodiment depicted in FIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

In an advance in the state of the art, a bumper spring assembly isconfigured with features to provide a substantially more robust designthat is resistant to being damaged by forces encountered during use. Forexample, the method of locking the headpiece to the mandrel or lockingthe cage nut to the mandrel using pins or set screws are much moresusceptible to being sheared as the mandrel recoils after being struckby a rapidly descending or ascending plunger. It is one of the principlefeatures and advantages of the present invention that the use of suchpins or screws is eliminated, thereby eliminating the risk of the pinsor screws becoming sheared in the presence of repeated recoil eventswhen the bumper spring assembly is subject to a rapidly descending orascending plunger. Other features of the bumper spring assembly areincluded to improve the flow of fluids through and around the bumperspring. The following description makes reference to various aspects ofthe attached drawings.

Introduction

The robust bumper spring assembly of the present invention includes fourcomponents, each with one or more distinct features that contribute tothe improved durability and performance of the assembly. Referring toFIG. 2 the four components are (1) a solid, one-piece mandrel/main shaftwith integral head piece and fishing neck at an upper or head endthereof; (2) a coil spring disposed on the main shaft portion of themandrel; (3) a cage disposed over the lower or tail end of the mandrelto retain the coil spring on the main shaft; and (4) an end nut (or cagenut) that threads onto the threaded tail end of the main shaft. The tailend of the mandrel is swaged to the installed end nut to permanently andsecurely retain the cage on the mandrel, thereby securing the fourcomponents together into a single unit that is highly resistant todeconstruction by conditions of use.

As will be described, each of these four structures has specific designfeatures that together produce a more robust bumper spring tool capableof superior performance and durability. Further, it will be apparentthat the present invention is capable of wide application to virtuallyall types of bumper springs having a variety of features, each intendedfor a specific purpose. However, bumper springs of all types mayadvantageously utilize the novel combinations of features described andclaimed herein. Accordingly, the disclosures herein apply to any bumperspring device that includes the combinations recited in the appendedclaims, regardless of whether a particular bumper spring includes ordoes not include other features in addition to the claimed combinations.

The solid, one-piece mandrel of the novel bumper spring includes twosections, a head piece and a main shaft, which are formed as a singlestructure that is referred to as the mandrel. Reference to either thehead piece or the main shaft is understood as reference to that sectionof the mandrel. The upper or head end of the one-piece mandrel disclosedherein includes the integral head piece that includes a fishing neck andfeatures to enhance stability of the bumper spring device and flow offluids moving past it. At its lower or tail end, just below the lowerend of the coil spring (when uncompressed) the OD of the main shaftportion of the mandrel is slightly reduced, forming a narrow externalshoulder “X” on the main shaft. The reduced portion includes externalthreads over a portion of its length to receive the internal threads ofan end nut. The tail end of the main shaft, called a forming end in somecontexts hereinafter, is bored to an ID slightly less than its reducedOD to receive a swaging tool die for locking the end nut onto the mainshaft.

The cage is a hollow cylindrical component that has an internal borethrough its length. The internal diameter (ID) of the upper portion ofthe cage that slides over the OD of the main shaft is slightly largerthan the OD of the main shaft by a nominal clearance value to enable itto move freely over the main shaft of the mandrel when the bumper springis absorbing the impact of a bypass plunger, for example. Below theupper portion of the cage, its ID expands to receive the end nut when itis threaded onto the main shaft. The junction of the enlarged ID of thecage and the nominal ID of the cage forms an internal shoulder “Y”within the cage. The end nut, when installed, forms a fixed stop nutthat bears against this internal shoulder “Y” to retain the cage againstor near the coil spring. The cage is free to slide for a limiteddistance along the main shaft as the coil spring is compressed whileabsorbing the momentum of a falling plunger against the upper or “head”end of the mandrel portion of the bumper spring assembly. When thusabsorbing the momentum of a plunger the lower portion of themandrel/main shaft moves downward though the cage against the tension inthe coil spring, and recoils upward as the coil spring returns towardits uncompressed state as the end nut contacts the internal shoulder “Y”of the cage.

The present invention combines several novel features to improve thedurability of the bumper spring assembly during its use in the bottom ofa well and to reduce its restriction to the flow of fluids past thebumper spring as it descends toward its installed position or duringproduction of the fluids from the formation served by the well.

The durability of the bumper spring is improved by (a) the use of aswaging operation to lock the end nut and the mandrel together. Further,(b) enlarging the OD of the headpiece portion of the mandrel slightlyabove the OD of the coil spring increases the stability of the bumperspring assembly in the well bore, thus reducing the wear of theheadpiece and increasing the longevity of the assembly. The flow offluids through and past the bumper spring assembly is improved by (c)machining shallow reliefs (flat or scalloped; if scalloped, having aslightly curved or concave cross section) along the outer surface of thehead around its diameter; (d) chamfering the lower OD of the headpieceinward to taper its profile, thus reducing the turbulence of fluidsaround the head of the mandrel; and (e) providing elongated passages orchannels formed from inside the cage outward and upward through the wallof the cage enabling fluids to flow through the cage from below and intothe well bore. These passages may preferably be further relieved at theoutlet to smooth the flow path. Further, the surfaces of the relievedpassages may preferably be formed with several narrow, closely-spacedcylindrical grooves or “egress sweeps” oriented parallel with each otherto reduce turbulence and improve flow of the fluids through the relievedpassages. All of these flow-enhancing features reduce drag and have beenfound to improve the flow of fluids from the 2.5 formation up to 20% ormore.

Regarding feature (a) in the preceding paragraph, the improvement to thedurability of the bumper spring by the use of a swaging operation tolock the end nut to the main shaft, there are two embodiments. In afirst embodiment, the length of the main shaft portion of the mandrelextends slightly past the lower end of the end nut when it is threadedonto the main shaft, and the protruding end of the main shaft is swagedin a machine operation—i.e., expanded or flared outward—to lock the endnut to the main shaft. In the flared version, the wall thickness of thebored end of the main shaft must be thin enough to permit the metal toproperly flare over the end nut to lock it in place.

In a second, preferred embodiment, the main shaft portion of the mandrelis altered so that the end nut butts against the narrow “X” shoulder onthe main shaft of the mandrel when the end nut is threaded thereon.Further, the lower end of the end nut is shortened so that it does notextend past the lower end of the main shaft when it is fully threaded onto the main shaft to butt against the narrow “X” shoulder. In addition,the ID of the end of the main shaft is reduced relative to the OD tothicken its wall and reduce the likelihood of cracks. Then, during theswaging operation, instead of flaring the end of the main shaft outwardover the end nut, the end of the main shaft is expanded just enoughduring the cold forming swaging operation to bear outward against theinternal threads of the end nut to lock the end nut into position on themain shaft between the narrow shoulder “X” and the swaged end of themain shaft. The effect of this swaging operation is to increase the IDof the main shaft's end slightly outward (due to the tapered shape ofthe cold forming die inserted into the ID of the main shaft as shown inFIG. 11). This preferred embodiment, because it locks the end nut atboth of its ends, more reliably secures the end nut to the main shaft sothat the assembly is much less likely to become loosened during use.

DETAILED DESCRIPTION

FIG. 2 illustrates an exploded view of an improved bumper springaccording to one embodiment of the present invention. Bumper springassembly 50 includes a one-piece mandrel 52 (including a main shaft 53)shown in the upper half of the drawing and having an integral headpiece60, a coil spring 54, and a cage 56 to be installed over the end of themandrel 52 opposite the headpiece 60 and secured with a cage (or end)nut 58. In the descriptions to follow, a portion of the mandrel 52 (andits main shaft 53) appear in detail views of the cage 56 in FIGS. 6 and7. Similarly, a portion of the mandrel 132 (and its main shaft 133)appear in detail views of the cage 136 in FIGS. 9 and 10. The headpiece60 includes a fishing neck 62, a plurality of longitudinal stabilizingridges 64 disposed around the outside diameter of the headpiece 60, aplurality of concentric flow reliefs 66, and a streamline taper (orexternal chamfer) 68. The headpiece 60 is a compound structureconfigured with several features to provide lateral stability to thebumper spring 50 in the bottom of the well or as it descends en routethereto. The stabilizing ridges 64 are configured as a plurality oflongitudinal strips of the OD of the headpiece 60 separated by theconcentric flow reliefs 66 disposed at equal radial intervals around thecircumference of the body of the headpiece 60. The concentric flowreliefs 66 may preferably include a plurality of longitudinal undercutsin the outermost diameter of the headpiece 60, alternately disposed withthe longitudinal stabilizing ridges 64 around the circumference of theheadpiece 60. The undercuts may be flat or curved in a concave manner,thereby forming trough-like passages for the flow of fluids that movepast the bumper spring 50 during use.

The mandrel 52 shown in FIG. 2 includes a tail end 48 and a threadedportion 70 on the OD of the tail end 48. In this description the tailend 4$ is also called a forming end 48, referring to its function duringa swaging operation to assemble the bumper spring. The bumper spring 54is preferably a coil spring formed of spring steel wire and having auniform coil pitch and closed and ground ends as shown in FIG. 2. As newmaterials are developed for spring applications, other materials havingequivalent characteristics may be suitable. Although no damping elementto absorb recoil of the bumper spring is shown in the drawings, someembodiments of the bumper spring device disclosed herein mayadvantageously employ a recoil damping mechanism for certainapplications.

Continuing with FIG. 2, the cage 56 includes egress or flow ports76—usually at least two disposed around the body of the cage 56—having arelieved channel 80 that includes a series of egress sweeps 82, whichare narrow, substantially parallel grooves formed to enhance the flow offluid through and away from the interior of the cage 56. The cageincludes an internal bore 84 for receiving the mandrel 52 and its tailend 48 therein after the coil spring 54 is assembled on to the mandrel52. An end nut 58 includes internal threads (not shown in this view)that enable it to be threaded onto the threads 70 of the tail end 48 tosecure the cage 56 on the mandrel 52. A pair of wrench flats 88 and atightening slot 94 are formed respectively in the cage 56 and the endnut 58 to facilitate the use of hand tools in tightening the end nut 58onto the mandrel 52. Further details of the configuration of the mandrel52, headpiece 60, spring 54, cage 56 and cage or end nut 58 aredescribed in other figures.

FIG. 3 illustrates a detail view of the head piece 60 disposed at theend of the embodiment of the one-piece mandrel 52 of FIG. 2, shownassembled with a portion of the coil spring 54. The mandrel 52 is, forexample, preferably machined from a single bar of alloy steel orstainless steel stock such that the headpiece 60 is formed as anintegral part of the mandrel 52. The head piece 60 includes a fishingneck 62 formed at the end. At the opposite end of the headpiece 60 thebody of the head piece 60 includes longitudinal portions—stabilizingridges 64—of the outer diameter surface of the head piece 60 separatedby a plurality of undercut flow reliefs 66 disposed around thecircumference of the head piece 60. This portion of the head piece formsa stabilizer. In a preferred embodiment, six of the flow reliefs 66(either flat or concavely curved), preferably of equal width, areseparated by equal-width longitudinal stabilizing ridges 64 in the outerdiameter of the head piece 60. The number and spacing of the flowreliefs 66 may be varied to suit a particular application. As mentionedpreviously, the profile of the reliefs 66 may be flat (“flats” 66) orcurved to form troughs (“troughs” 66). To reduce damage to the coilspring 54 and the mandrel 52, the outer diameter of the head piece 60,which is coincident with the surfaces of the stabilizing ridges, issized to be slightly larger than the outer diameter of the coil spring54 to provide stability within the well tubing by reducing the amount ofside-to-side deflection during descent of the bumper spring 50. At thelower end of the head piece adjacent to the spring 54, the edge 68 ofthe head piece is chamfered. Together with the flow relief fiats 66, thechamfered edge 68 enhances the flow of fluids past the bumper spring byreducing drag as the bumper spring descends.

The one-piece or unitary head piece portion of the bumper springillustrated in FIG. 3 provides superior strength and durability of theheadpiece/mandrel combination as compared with a separate headpiece thatis attached to the end of the mandrel with respective internal andexternal threads and then secured with pins or set screws passed throughthe two separate parts. The traditional method of locking the headpieceto the mandrel using pins or set screws is much more susceptible tobeing sheared as the mandrel recoils after being struck by a rapidlydescending or ascending plunger. It is one of the principle features andadvantages of the present invention that the use of such pins and screwsis avoided, thereby eliminating the risk of the pins or screws becomingsheared in the presence of repeated recoil events when the bumper springassembly is subject to a rapidly descending plunger.

FIG. 4 illustrates a detailed view of a portion of the cage 56 of theembodiment of FIG. 2 to depict the flow ports 76 disposed around thebody of the cage 56. The cage 56, being hollow to receive the mandrel 52includes the flow ports 76—openings between the inside and outside ofthe cage—to provide a path for the flow of fluids through the bumperspring assembly 50 when it is disposed at the bottom of a well or whileit is descending through the well bore tubing. The flow ports 76 areusually disposed at equal intervals around the body of the cage 56, andinclude a relieved channel 80 for each flow port 76 that includes aseries of egress sweeps 82, which are narrow, substantially parallelgrooves formed in each relieved channel 80 to enhance the flaw of fluidthrough and away from the interior of the cage 56.

FIGS. 5A and 5B depict assembled versions of the embodiment of FIGS. 2and 3 to show the relationship of the various components. FIG. 5Aillustrates a side view of the assembled bumper spring 100 of theembodiment of FIGS. 2 and 3, while FIG. 5B illustrates a cross sectionview of the assembled bumper spring 100 of FIG. 5A to show the internalrelationships of the components described for FIGS. 2, 3 and 4. All ofthe components of FIGS. 5A and 5B are identified with the same referencenumbers as previously.

FIG. 6 shows a cross section view of the lower end of the bumper springassembly 100 of FIG. 5B prior to a swaging or cold-forming operationperformed with a swaging tool (to be described) under very highpressure. FIG. 7 depicts the configuration of the bumper spring assembly100 after the swaging operation. Shown in FIG. 6 is a bore 72 formed inthe tail end 48 of the mandrel 52. The bore 72 is concentric with thelongitudinal centerline of the mandrel 52. Also shown in FIG. 6 is ashoulder 78 (the “Y” shoulder described previously) formed inside thecage 56. The shoulder 78 is provided for the end nut 58 to hear againstwhen the end nut 58 is fully threaded onto the mandrel 52, to positionthe cage 56 against the lower end of the spring 54. The spring 54 isthus confined between the cage 56 and the integral head piece 60 (seeFIGS. 5A and 5B). Reference number 70 indicates the locus of thethreaded joint between the tail end of the mandrel 48 and the end nut58. The end nut 58 retains the cage 56 on the mandrel 52.

FIG. 7 depicts, in cross section, the same assembly as FIG. 6 exceptthat the tail end 48 of the mandrel 52 has been swaged, by cold formingusing a special die, to flare the tail end 48 outward against the end ofthe mandrel 52, thereby producing the flared end 74. This configurationprovides a positive locking of the cage nut 58 onto the threaded end ofthe mandrel 52, thereby creating a unitary structure that is moredurable, i.e., resistant to loosening or disassembly, than the use ofpins or set screws to lock these two components together. As is wellknown, pins and set crews are susceptible to being sheared as themandrel recoils after being struck by a rapidly descending plunger.

FIG. 8 illustrates a side view of a swaging tool 110 (or die 110) foruse in cold forming the tail end 48 (as shown in FIG. 7) when assemblingthe bumper spring 54, bumper spring cage 56, and cage or end nut 58 tothe mandrel 52 according to the embodiment of FIGS. 2 and 5. The swagingtool or die 110 is machined at the end (to the left in the figure) toform the flared end 74 of the bore 72 in the end of the mandrel 52. Thedetails of the specific configuration of the swaging tool 110 and itsbody 112 include a fillet radius 114, a swaging taper 116, a lead-inangle 118, and a flanging face 120. The die 110 is thus shaped to flarethe end of the mandrel 52 so that it can firmly and securely trap thecage nut 58 on the mandrel 52 smoothly and without cracking the metal ofthe mandrel 52.

To perform the swaging operation the swaging tool 110 is installed in apress (not shown) having a capacity of at least 14 tons. The machinedend (114 etc.) of the swaging tool 110 is inserted into the bore 72 ofthe forming or tail end 48 of the mandrel 52 after the bumper spring 54,bumper spring cage 56 and cage nut 58 are installed on the mandrel 52.Under sufficient pressure exerted by the press, the steel material atthe forming end 48 of the mandrel 52 flows outward in the cold formingprocess to form the flared end 74 of the mandrel 52 against the cage nut58 as shown in FIG. 7.

FIGS. 9, 10, and 11 illustrate cross section and side views of analternative (or “second”) embodiment to the embodiment depicted in FIGS.6, 7, and 8 that as been found to be even more effective in securing thecomponents of the bumper spring 50 from loosening. The embodiment shownin FIGS. 9 and 10 is similar to the embodiment depicted in FIGS. 2through 7 except for the structure of the joint that secures the end nutto the mandrel.

FIG. 9 shows a cross section view of the lower end of a secondembodiment of the bumper spring assembly 100 of FIGS. 5A and 5B prior tothe swaging or cold-forming operation performed with the swaging tool170 (to be described) of FIG. 11 under very high pressure. FIG. 10depicts the configuration of the second embodiment of the bumper springassembly 100 after the swaging operation. Shown in FIG. 9 is a bore 152formed in the tail end 148 of the mandrel 132. The bore 152 isconcentric with the longitudinal centerline of the mandrel 132. Thespring 134 is shown surrounding the mandrel 132. Also shown in FIG. 9 isa cage shoulder 158 (the shoulder “Y” described previously) formedinside the cage 136. The cage shoulder 158 is provided for the end nut138 to bear against when the end nut 138 is fully threaded onto themandrel 132, to position the cage 136 against the lower end of thespring 134. The spring 134 is thus confined between the cage 136 and theintegral head piece 60 (see FIGS. 5A and 5B).

Continuing with FIG. 9, in the second embodiment shown in FIG. 9 theforming end 148 of the mandrel 132 is configured slightly differentlythan the mandrel 52 depicted in FIG. 6. This difference is that themandrel shoulder “X” as shown in FIG. 6 is extended to the position 168shown in FIG. 9, eliminating the clearance gap 90 (See FIG. 6) so thatthe end nut 138 butts against both the cage nut shoulder 158 and themandrel shoulder 16$ when the end nut 138 is fully threaded onto themandrel 132. Reference number 150 indicates the locus of the externalthread surrounding the outer diameter of the bored out portion 152 ofthe tail end 148 of the mandrel 132 and the internal threads of the endnut 138 that is installed on the tail end 148 of the mandrel 132. Theend nut 138 is used to secure the cage 136 onto the mandrel 132. The endnut 138 is formed with a nominal ID that is threaded to match thethreads on the tail end 148 of mandrel 132 and a 45 degree conicalexpansion of the nominal ID from the 45 degree conical expansion to alarger ID to the end of the end nut 138, as shown in FIG. 9.

FIG. 10 depicts, in cross section, the same assembly as FIG. 9 (i.e., ofthe second embodiment of the bumper spring assembly 100 of FIG. 5B)except that the forming end 148 of the mandrel 132 has been swaged, bycold forming using the special die 170 shown in FIG. 11, to expand thedistal portion of the tail end 148 outward against the inside wall ofthe cage or end nut 138. As thus expanded, the outer surface of themandrel 132 at the end of the threaded portions of the end nut 138 andmandrel 132 deforms the threads 150 to prevent them from loosening. Thisconfiguration provides a positive locking of the end nut 138 onto thethreaded end of the mandrel 132, thereby creating a unitary structurethat is more durable, i.e., resistant to loosening or disassembly, thanthe use of pins or set screws to lock these two components together. Asis well known, pins and set crews are susceptible to being sheared asthe mandrel recoils after being struck by a rapidly descending plunger.

FIG. 11 illustrates a side view of a swaging tool 170 (or die 170) forcold forming the tail end 148 of the second embodiment depicted in FIGS.9 and 10 when assembling the bumper spring 134, bumper spring cage 136,and cage or end nut 138 to the mandrel 132 according to the embodimentof FIGS. 2 and 5. The swaging tool or die 170 is machined at the end (tothe left in the figure) to form the expanded end 154 of the bore 152 inthe end of the mandrel 132. The details of the specific configuration ofthe swaging tool 170 and its body 172 include sections of the tool 170for forming respectively the guide diameter 174 (for aligning the toolwith the bore 152), the swage/lead-in angle 176 (for forming thetransition zone 153), the swaging diameter 178 (for expanding theforming end 148 of the mandrel 132 to the expanded end 154), and thestop face 180 (for controlling the penetration depth of the swaging tool170).

To perform the swaging operation the swaging tool 170 is installed in apress (not shown) having a capacity of at least 14 tons, The machinedend (174 etc.) of the swaging tool 170 is inserted into the bore 152 ofthe forming end 148 of the mandrel 132 after the bumper spring 134,bumper spring cage 136 and cage nut 138 are installed on the mandrel132. Under sufficient pressure exerted by the press, the steel materialat the forming end 148 of the mandrel 132 flows outward to form theexpanded end 154 of the mandrel 132 against the cage nut 138 as shown inFIG. 10. The die is shaped and the press is operated so that the diedoes not bottom out or become stuck in the bore 152.

While the invention has been shown and described in only one of itsforms, it is not thus limited but is susceptible to various changes andmodifications without departing from the spirit thereof. For example, itis well known that bumper springs are available in numerousconfigurations, designed for use in a variety of conditions andapplications in the down-hole well environment. As such, a bumper springdevice may have a variety of features, each intended for a specificpurpose. However, bumper springs of all types may have a number offeatures or characteristics in common, including the novel combinationsof features described and claimed herein. Accordingly, the disclosuresprovided herein apply to any bumper spring device that includes thecombinations of features recited in the following claims, regardless ofwhether a particular bumper spring includes or does not include otherfeatures in addition to the claimed combinations. Examples of such otherfeatures may include but are not limited to seating cups or seals, latchdown devices, tubing stops or collars, no-go devices or features ofseating or landing nipples, and the like.

What is claimed is:
 1. A method for locking an internally threaded endpiece to a mandrel for a down hole tool, without using pins or setscrews, comprising: boring a distal portion of a solid body of themandrel to a predetermined diameter and depth such that at least aportion of the solid body is not bored; forming an external threadaround the bored distal portion of the mandrel for receiving theinternally threaded end piece; installing the internally threaded endpiece on the externally thread of the bored distal portion of themandrel; and expanding the outermost end of the bored distal portion ofthe mandrel outward in a cold forming process against proximate threadsof the internally threaded end piece to lock threads of the internallythreaded end piece to the bored distal portion of the mandrel.
 2. Themethod of claim 1, wherein the step of boring comprises the step of:boring the distal portion of a solid body of the mandrel to receive aswaging die formed to provide the predetermined diameter and depth. 3.The method of claim 1, wherein the step of forming comprises the stepof: forming the external thread to the same pitch and thread profile ofthe threads of the internally threaded end piece.
 4. The method of claim1, wherein the step of installing comprises: advancing the internallythreaded end piece to align its outer end with a prescribed referencealong the bored distal portion of the mandrel.
 5. The method of claim 1,wherein the step of expanding comprises the steps of: securing themandrel in a supporting head stock on a cold forming bench; and guidinga swaging tool into the bored distal portion of the mandrel under apredetermined pressure and advance until the expansion of the boreddistal portion of the mandrel engages and deforms the proximate threadsof the internally threaded end piece to lock the end piece to the boreddistal portion of the mandrel.
 6. A method for locking a head piece ortail piece to a mandrel for a down hole tool, without using pins or setscrews, comprising: boring a distal portion of a solid body of themandrel to a predetermined diameter and depth such that at least aportion of the solid body is not bored; forming an external threadaround the bored distal portion of the mandrel; assembling the headpiece or tail piece over the bored distal portion of the mandrel;installing an internally threaded end nut or sealing nut on theexternally thread of the bored distal portion of the mandrel; andexpanding the outermost end of the bored distal portion of the mandreloutward in a cold forming process against proximate threads of theinternally threaded end nut or sealing nut to lock threads of theinternally threaded end nut or sealing nut to the bored distal portionof the mandrel and to lock the head piece or tail piece to the mandrel.7. The method of claim 6, wherein the step of boring comprises the stepof: boring the distal portion of a solid body of the mandrel to receivea swaging die formed to provide the predetermined diameter and depth. 8.The method of claim 6, wherein the step of forming comprises the stepof: forming the external thread to the same pitch and thread profile ofthe threads of the internally threaded end nut or sealing nut.
 9. Themethod of claim 6, wherein the step of installing comprises the step of:advancing the internally threaded end nut or sealing nut to align itsouter end with a prescribed reference along the bored distal portion ofthe mandrel.
 10. The method of claim 6, wherein the step of expandingcomprises the steps of securing the mandrel in a supporting head stockon a cold forming bench; and guiding a swaging tool into the boreddistal portion of the mandrel under a predetermined pressure and advanceuntil the expansion of the bored distal portion of the mandrel engagesand deforms the proximate threads of the end nut or sealing nut to lockthe end nut or sealing nut to the bored distal portion of the mandreland to lock the head piece or tail piece to the mandrel.